1. Field of the Invention
The present invention relates to a source driver for a display panel such as an active matrix liquid crystal panel and a drive control method therefor.
2. Description of the Related Art
It is known that AC driving is required in order to assure long-term reliability of a liquid crystal panel. Accordingly, in a conventional active matrix liquid crystal display device, the polarity of the drive voltage applied by a source driver between the electrodes of a liquid crystal element of each cell (pixel) of the liquid crystal panel is inverted for each frame of the image signal (see Japanese Laid-open Patent Application No. 2002-108303). Rather than simultaneously setting all cells of the liquid crystal panel to the same polarity, there is used a dot inversion drive-scheme, in which the cells in adjacent columns and rows become inverted in polarity, or a two-line dot inversion method, in which every two lines are inverted in the rows. In order to generate such a positive polarity and negative polarity a drive voltage, the source driver is provided with two differential amplifiers (operational amplifiers) using odd-numbered source signal lines and even-numbered source signal lines of a liquid crystal panel, e.g., two N-channel MOS transistors and two P-channel MOS transistors, and drive voltages of mutually opposite polarity that reverse polarity for each frame are generated at the output terminal of the source driver connected to the odd-numbered source signal line and the output terminal of the source driver connected to the even-numbered source signal line. However, an offset voltage component is ordinarily included in the drive voltages because of characteristic differences in manufacturing the transistors of the two differential amplifier circuits.
Nonuniform display occurs or image quality is otherwise negatively affected when an offset voltage is included in the drive voltage. Therefore, the offset voltage is canceled (see Japanese Laid-open Patent Application No. 2002-108303). For example, the drive voltage outputted to the odd-numbered output terminals is offset in the following manner: one of two transistors of one differential amplitude circuit of two differential amplitude circuits is switched on in the first frame of four consecutive frames, and a positive polarity drive voltage that includes an offset voltage +A is generated; one transistor of the other differential amplifier circuit is switched on in the second frame, and a negative polarity drive voltage that includes an offset voltage +B is generated; the other transistor of one of the differential amplifier circuits is switched on in the third frame, and a positive polarity drive voltage that includes an offset voltage −A is generated; and the other transistor of the other differential amplifier circuit is switched on in the fourth frame, and a negative polarity drive voltage that includes an offset voltage −B is generated. The offset voltages in the four frames cancel each other in the manner of +A+B−A−B=0.
In the conventional liquid crystal display device described above, a vertical synchronization signal of an image signal is required to be inputted to a source driver in order to generate an AC drive control signal or another control signal for accurately detecting between-frames and controlling the connecting and switching between odd- and even-numbered output terminals and the outputs of two differential amplifier circuits. However, the vertical synchronization signal is sometimes not fed to the source driver depending on the existing liquid crystal panel, and there is difficulty in accurately detecting between-frames.